CMR is a rapidly growing telecommunications system. The typical CMR system includes a multiplicity of cells, such as indicated in FIG. 1. A particular geographic area is subdivided into a multiplicity of subareas, with each of the subareas being serviced by a stationary transmitter/receiver setup. The cells are set up to carry signals to and from mobile units M in the range of the cell. If one cell becomes too crowded, it can be divided into smaller cells, by a process known as cell splitting. As can be seen from FIG. 1, any particular geographic area can become quite complicated with cells overlapping each other, and overlapping cells of other neighboring cellular systems. It is noted that the term "cellular" is intended to be a term of convenience, and is not intended to be limiting. The present disclosure is intended to encompass any communication system in which an overall area can be divided into one or more subareas such as shown in FIG. 1.
A typical CMR set up is indicated in FIGS. 2 through 7, and will be described so an understanding of the problem to which this invention is directed can be obtained.
FIGS. 2, 3 and 4 show a typical cellular telephone unit 2 having a unique mobile identification number stored in a suitable location such as an electrically erasable programmable read-only memory (not shown). Telephone units of this kind are known to those skilled in this art, and thus will not be described in detail.
The telephone unit 2 includes a handset 4 having a keypad 5 as well as a speaker 6 and a microphone 7. A transceiver 8, ordinarily built into the telephone unit 2, exchanges signals via an antenna 10 with a mobile telecommunications switching office or MTSO 12 via a cell site 14. A duplexer 15 connects the antenna to the transceiver. The cell site 14 includes an antenna 16 connected to a control terminal 17 via a transceiver 18. The cell 14 is connected to the MTSO via a transmission link 20.
Referring to FIGS. 4, 4A and 5, the operation of the CMR can be understood. The mobile unit M moves about the geographic areas covered by the various cells as indicated in FIG. 1. As that mobile unit moves about, it decodes the overhead message control signals generated by various cell site control channels. The mobile unit locks onto the cell site that is emitting the strongest signal. The mobile unit rescans channels periodically to update its status. If, for example, a fixed-position land-based telephone T is used to call the mobile unit, a signal is sent via landlines L, to the central office CO of a public/switched telephone system (PTSN) 12A. This system then utilizes the switching network SN associated therewith to call the MTSO 12 via a transmission link L1. The MTSO then utilizes its own switching network and generates a page request signal to all cell sites via transmission links, such as the transmission link 20. The cell site which has been notified of the presence of the mobile unit M sends a signal back to the MTSO via the landlines alerting the MTSO of the presence of the mobile unit. The MTSO then orders the mobile unit, via the notifying cell site, to tune to an assigned channel and receive the call. Billing and other business information are recorded in the MTSO at this time.
On the other hand, during call origination, the mobile unit rescans the control channels to determine which is the best server based on signal strength. Upon selecting the best server, the mobile unit transmits call site information on the control channel receive frequency and then receives a voice channel to tune to if the mobile unit is authorized to place a call.
As the mobile unit moves, the signal strength between that mobile unit and the originating cell site changes, and perhaps diminishes. Since signal strength is an inverse function of the square of the distance between the mobile unit and the cell site, signal strength can change rapidly and drastically as the mobile unit moves with respect to the cell site and therefore must be monitored closely. Moreover, signal strength can be strongly affected by terrain, environmental conditions as well as interference from other sources. The MTSO has a signal strength table ST, and signal strength from the mobile unit is constantly compared to acceptable signal strength levels in the table. Such a table can be located in each cell site if desired.
Should signal strength diminish below a preset range, the MTSO generates a "locate request" signal to all cell sites that neighbor the original cell site. Each of such neighboring cell sites receiving a signal from the mobile unit signals the MTSO, and the signal strength from such neighboring cell sites are checked against the signal strength table. The MTSO makes a decision as to which cell site should control the call, and notifies the original cell site to order the mobile unit to retune to a voice channel of the new cell site.
As soon as the mobile unit retunes, the mobile unit completes the call via the new cell site channel. This transfer of control is known as a handoff.
While this method of making switching decisions has worked well in the past, the growth and sophistication of the cellular industry has resulted in severe drawbacks to this method. First, due to uneven terrain, unpredictable environmental conditions, interference and the like, many cellular companies have been required to construct numerous cell sites. These cells often overlap neighboring cell sites and provide redundant coverage. This is extremely expensive, not only from the standpoint of construction costs, but due to monitoring and staffing costs as well. Even at this, conditions can change so rapidly that coverage may still be inconsistent.
Still further, due to idiosyncrasies in terrain and environment, a mobile unit may use a cell that is located far from the mobile unit rather than a cell located immediately adjacent to that mobile unit. Hilly terrain is a common example of this problem. While this may not be a technical problem, it is important because a cellular company cannot assess long distance charges and/or message units to the calls. This deprives the cellular company of income that it could otherwise receive and customers of optimum service. Communities are also deprived of tax income that might be assessed against such calls as well.
Still further, since only signal strength is used to make switching decisions, the location of a caller is not ascertainable. This could be important in keeping track of calls.
Other problems that have been experienced in such cellular systems include the inability to completely control the cell site transmit signal, crosstalk noise interference, dropped calls, overlap and an inability to adequately service areas with undulating terrain without infringing the borders of other cellular territories.
Therefore, there is a need for a cellular system that can provide consistently high quality service, yet can do so with a minimum number of cell sites in a particular geographic area. Still further, there is a need for a cellular system that can accurately assess charges for all CMR services including message units for calls covering a certain distance within the geographic area.